Cars have an immense amount of sensors. A check engine light will alert the owner that the computer has identified a sensor that is out of tolerance. What you need to do is scan the codes. Sometimes Autozone or another parts store will use a code reader on your vehicle for free.

Old transmissions were all self contained mechanical control using hydraulics. Transmissions are now electronically controlled and require a computer to control. There could be many things causing the malfunction. Read the codes. It will likely tell you the sensor that is out of tolerance. Either the sensor is malfunctioning or the sensor is detecting a condition that is out of spec.